Among modern medicine's great achievements is the development and successful use of antimicrobials against disease-causing microbes. Antimicrobials have saved numerous lives and reduced the complications of many diseases and infections. However, the currently available antimicrobials are not as effective as they once were.
Over time, many microbes have developed ways to circumvent the anti-microbial actions of the known antimicrobials, and in recent years there has been a worldwide increase in infections caused by microbes resistant to multiple antimicrobial agents. With the increased availability and ease of global travel, rapid spread of drug-resistant microbes around the world is becoming a serious problem. In the community, microbial resistance can result from nosocomial acquisition of drug-resistant pathogens (e.g., methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant Enterococci (VRE)), emergence of resistance due to use of antibiotics within the community (e.g., penicillin- and quinolone-resistant Neisseria gonorrheae), acquisition of resistant pathogens as a result of travel (e.g., antibiotic-resistant Shigella), or as a result of using antimicrobial agents in animals with subsequent transmission of resistant pathogens to humans (e.g., antibiotic resistant Salmonella). Antibiotic resistance in hospitals has usually resulted from overuse of antibiotics and has been a serious problem with MRSA, VRE, and multi-drug resistant Gram-negative bacilli (MDR-GNB) (e.g., Enterobacter, Klebsiella, Serratia, Citrobacter, Pseudomonas, and E. coli). In particular, catheter-related blood stream infections by bacteria and skin and soft tissue infections (SSTIs) are becoming an increasing problem.
Bacteria, viruses, fungi, and parasites have all developed resistance to known antimicrobials. Resistance usually results from three mechanisms: (i) alteration of the drug target such that the antimicrobial agent binds poorly and thereby has a diminished effect in controlling infection; (ii) reduced access of the drug to its target as a result of impaired drug penetration or active efflux of the drug; and (iii) enzymatic inactivation of the drug by enzymes produced by the microbe. Antimicrobial resistance provides a survival advantage to microbes and makes it harder to eliminate microbial infections from the body. This increased difficulty in fighting microbial infections has led to an increased risk of developing infections in hospitals and other settings. Diseases such as tuberculosis, malaria, gonorrhea, and childhood ear infections are now more difficult to treat than they were just a few decades ago. Drug resistance is a significant problem for hospitals harboring critically ill patients who are less able to fight off infections without the help of antibiotics. Unfortunately, heavy use of antibiotics in these patients selects for changes in microbes that bring about drug resistance. These drug resistant bacteria are resistant to our strongest antibiotics and continue to prey on vulnerable hospital patients. It has been reported that 5 to 10 percent of patients admitted to hospitals acquire an infection during their stay and that this risk has risen steadily in recent decades.
In view of these problems, there is an increasing need for novel antimicrobials to combat microbial infections and the problem of increasing drug resistance. A renewed focus on antimicrobial drug discovery is critical as pathogens are developing resistance to available drugs.
Synthetic compounds have thus far failed to replace natural antibiotics and to lead to novel classes of broad-spectrum compounds, despite the combined efforts of combinatorial synthesis, high-throughput screening, advanced medicinal chemistry, genomics and proteomics, and rational drug design. The problem with obtaining new synthetic antibiotics may be related in part to the fact that the synthetic antibiotics are invariably pumped out across the outer membrane barrier of bacteria by Multidrug Resistance pumps (MDRs). The outer membrane of bacteria is a barrier for amphipathic compounds (which essentially all drugs are), and MDRs extrude drugs across this barrier. Evolution has produced antibiotics that can largely bypass this dual barrier/extrusion mechanism, but synthetic compounds almost invariably fail.